The present invention relates to an electric signal producing system for producing various types of information by sampling a received signal and, more particularly, to an electric signal producing system in which its resolution and accuracy are improved.
FIG. 10 is a block diagram showing a conventional signal producing system, for example, applied to a magnetic resonance apparatus. In FIG. 10, a body 2 to be examined is disposed in a magnet 1 for generating a static magnetic field in a direction Z.
A gate modulation amplifier 4 for amplifying a radio frequency (RF) wave A as a pulse is connected to an oscillator 3 for generating the RF wave A, a transmission/reception switching coupling circuit 5 is connected to the gate modulation amplifier 4, an RF coil 6 disposed in the magnet 1 is connected to the coupling circuit 5, an amplifier 7, to which a reception signal B from the RF coil 6 is inputted, is connected to the coupling circuit 5, a 0.degree. phase detector 8 and a 90.degree. phase detector 10 are connected to the amplifier 7, a 90.degree. phase shifter 9 is inserted between the oscillator 3 and the phase detector 10, A/D converters 11 and 12 are respectively connected to the phase detectors 8 and 10, a memory 13 is connected to the A/D converters 11 and 12, and a computer 14 having a calculating function for controlling an entire apparatus is connected to the memory 13.
An oblique magnetic field power source 15 is connected to the computer 14, an oblique magnetic field coil 16 disposed in the magnet 1 is connected to the oblique magnetic field power source 15, and a display unit 17 for displaying various types of information is connected to the computer 14.
Next, the operation of a conventional signal producing system shown in FIG. 10 will be described with an example of the case that an NMR signal is produced from the body 2 to be inspected as a reception signal B.
The RF wave A from the oscillator 3 is converted by the gate modulation amplifier 4 to pulses, which is further applied through the coupling circuit 5 to the RF coil 6, and emitted as RF pulses to the body 2 to be inspected. Thus, atomic nuclei in the body 2 to be inspected go into an excited state, and a reception signal B is generated when they return to an original thermal balanced state.
The reception signal B is detected by the RF coil 6, and inputted to the phase detectors 8 and 10 through the coupling circuit 5 and the amplifier 7. At this time, since the phase detector 8 uses the RF wave A as it is as a detecting reference signal and the phase detector 10 uses the RF wave A in which its phase is displaced by 90.degree. through the phase shifter 9, the reception signal B is quadrature-detected.
Then, the reception signal B is sampled by A/D converters 11 and 12 at a predetermined sampling frequency fs corresponding to a necessary frequency band, and stored as digital data D.sub.1 and D.sub.2 in the memory 13.
Subsequently, the digital data D.sub.1 and D.sub.2 are transferred to the computer 14, and analyzed and calculated, for example, by a Fourier conversion method or a maximum entropy method or the like to obtain a signal for producing desired distribution information, such as a frequency spectrum or an image or the like.
When the magnetic resonance apparatus is of an MRI (Magnetic resonance imaging apparatus), in order to two-dimensional Fourier-convert the digital data D.sub.1 and D.sub.2 of the reception signal B to reconstruct an image, the computer 14 controls the oblique magnetic field power source 15 to generate an oblique magnetic field of a predetermined sequence from the oblique magnetic field coil 16, thereby providing position information of atomic nucleus spin in the reception signal B.
FIG. 11 is a pulse sequence diagram for producing a reception signal B used for a Fourier conversion imaging method disclosed, for example, in British Patent No. 2,079,946.
In this case, an RF pulse is of a 90.degree. pulse, of oblique magnetic fields Gx, Gy and Gz of orthogonal 3 axes (X, Y and Z) directions, the Gz is a slice magnetic field, Gx is a phase encoding magnetic field, and Gy is a frequency encoding magnetic field. The Fourier conversion imaging method is described in detail, for example, on pages 54 to 56 of "NMR Medical" (issued by Maruzen Co., Ltd., edited by Nuclear Magnetic Resonance Medical Institute of Technology), and the description will be omitted.
As shown in FIG. 11, the position information of the excited atomic nucleus spin is applied to the reception signal B received during a sampling period by applying the phase encoding magnetic field Gx and the frequency encoding magnetic field Gy. Accordingly, the distribution image of the atomic nucleus spin is reconstructed by the Fourier conversion imaging method in accordance with the signal obtained by Fourier-converting the digital data D.sub.1 and D.sub.2, and this image is displayed on the display unit 17.
In order to produce a magnetic resonance image having N.times.M pixels by the Fourier conversion imaging method, it is necessary to systematically vary the phase encoding magnetic field Gx N times (refer to a broken line), and to sample N types of reception signals B by the A/D converters 11 and 12 N times. Then, the obtained N.times.N data (u, v) are two-dimensional Fourier-reverse converted to produce images f (x, y), where u corresponds to a phase encoding direction, v corresponds to a time sampling direction, u=1, 2, . . . , N, and v=1, 2n, . . . , N. Here, a reception signal row obtained by Fourier-converting the digital data D.sub.1 and D.sub.2 is represented by the spatial frequency expression (from the result that the image is two-dimensional Fourier-converted) of the magnetic resonance image.
Generally, it is understood as described, for example, in "The Television Society Journal" (No. 8, Vol 37, in 1983) that the spatial frequency components of the image is concentrated to low frequency components. Accordingly, an energy is mostly concentrated at the d.c. component of the spatial frequency, and reduced exponentially toward high frequencies. The actually obtained reception signal B indicates a similar trend to this.
FIGS. 12(a) and 12(b) show ideal waveforms of one line passing the d.c. component of the reception signal B (when the phase encoding magnetic field Gx is zero), wherein FIG. 12(b) is a waveform diagram showing the enlarged central portion of the waveform in FIG. 12(a) enlarged in a time base (lateral) axis direction, corresponding, for example, to the digital data D.sub.1 and D.sub.2 of a point N (=256) when the quantizing errors of the A/D converters 11 and 12 are not presented. FIG. 13 is a waveform diagram of a signal obtained by one-dimensional Fourier-converting the length of the sampling point number N for the digital data D.sub.1 and D.sub.2. In this case, its signal waveform becomes ideal rectangular, the central part of the rectangular portion corresponds to a d.c. frequency, the both sides of the central part correspond to low frequencies, and the parts separated from the central part correspond to high frequencies. The Fourier conversion is of complex Fourier conversion, the waveform of a complex number section similarly exists, but only a real number section will be described for the simplification of the description.
FIGS. 14(a) and 14(b) are waveform diagrams of the digital data D.sub.1 and D.sub.2 obtained actually corresponding to FIGS. 12(a) and 12(b), illustrating the case that the effective bit number corresponding to its amplitude is 4 bits.
In this case, the A/D converters 11 and 12 are so set that the maximum value of the reception signal B become the maximum value of the digital data D.sub.1 and D.sub.2 so as not to overflow at the time of A/D conversion. Accordingly, the peak signal Bp of the central part is converted as the maximum data of the A/D converters 11 and 12, and the small amplitude components on the both sides of the peak signal Bp are buried in the (quantizing error) range of the quantizing step to lose information.
As described above, the result that the digital data D.sub.1 and D.sub.2 in which information of high frequency components are lost are Fourier-converted becomes as shown in FIG. 15, the rise and fall of the rectangular portion become obtuse as compared with the ideal waveform (FIG. 13), and a noise due to the quantizing error is generated.
When the magnetic resonance apparatus is not the MRI but a spectroscopy for producing a frequency spectrum, its reception signal B is equal to a one-dimensional signal on a line passing a d.c. component of the signal obtained by a Fourier conversion imaging method. In this case, the digital data D.sub.1 and D.sub.2 are, for example, one-dimensional or two-dimensional Fourier-converted as they are.
As mentioned above, when the reception signal B is of the NMR signal, the image constitution or frequency spectral analysis can be performed in accordance with the reception signal B.
Then, a signal processing method for directly producing a spin-spin moderating time T.sub.2 * from the time component data of a reception signal B will be described with an example of the case that the reception signal B is of an FID (free induction damping) signal.
In this case, in the pulse sequence in FIG. 11, the oblique magnetic fields Gz, Gx and Gy are not applied at all, but an FID signal immediately after the 90.degree. RF pulse is applied is received as a reception signal B.
FIG. 16 is a waveform diagram of digital data D.sub.1 and D.sub.2 of a reception signal B, for example, at N (=256) points when the quantizing errors of the A/D converters 11 and 12 are not presented. In this case, since the reception signal B is not received during the application of the RF pulses, it becomes a waveform received from an initial amplitude M.
FIG. 17 shows an ideal envelope obtained from the waveform in FIG. 16, and the spin-spin moderating time T.sub.2 * of a body 2 to be inspected is obtained as a time x until the initial amplitude M is attenuated to (1/e).
FIG. 18 is a waveform diagram of digital data D.sub.1 and D.sub.2 obtained actually corresponding to FIG. 16, illustrating the case that the effective bit number corresponding to the maximum initial amplitude M is 4 bits similarly to the above description. In this case, the data of the small amplitude is also buried in the quantizing error.
Therefore, the envelope obtained from the waveform in FIG. 18 becomes as shown in FIG. 19, and it is understood that the error of the spin-spin moderating time T.sub.2 * obtained from .rho.=T.sub.2 * is large.
Since the conventional signal producing system so converts in the A/D converters 11 and 12 that the maximum value of the reception signal B become the maximum value of the digital data D.sub.1 and D.sub.2 as described above, most reception signals B are buried in the quantizing error of the A/D converters 11 and 12 and not detected, and the system has a problem that the resolution of the frequency spectral analysis or the timing information producing accuracy is deteriorated.